Phase modulation



Jan; 5, 1937.

H. E. GoLDsTlN PHASE MODULATION Filed `July 11, '1935 4 Sheets-Sheet 1 INVENTOR HALLAN EUGENE GOLDSTINE ATTORNEY Jan. 5, 1937. H. E. GoLDsTlNE 2,057,082

f PHASE MoDULATIoN Filed July 11, 1935 .A 4 sheets-sheet 5 Y lNvENToR HALLN EUGENE GLDSTINE ATTORNEY Jan. 5, 1937.Y

H. E'. GOLDSTINE PHASE MODULATION Filed July 11, 1933 'fari/m41 4 sheets-sheet 4 -IIIII lNvEN-roR HALLAN EUGENE eoLosTlNE ATTORNEY Patented Jan. 5, 1937 Y t UNITED STATES PATENT OFFICE PHASE MODULATION Hallan Eugene Goldstine, Rocky Point, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 11, 1933, Serial No. 679,916

16 Claims. (Cl. 179-171) Ihis invention relates to the art of signalling The character of the phase modulated Wave by means of high frequency oscillations, the may be altered as to amplitudaor phase or character of which varies in accordance with frequency, or in one or more of said respects. signals, and more in particular with a method In the usual type of'phase modulation transof and means for displacing the phase of high mitters, elaborate equipment is needed to profrequency oscillations in a correct relationship duce quality of modulation. to an applied signal voltage, thereby producing An object of the present invention is to pro- `a signal so modulated that intelligent transmisvide a device which is simple in nature and sion may be made by using phase modulation. structure and which modulates a large amount In the usual method of modulating the outof power output with good quality modulation 10 put current of a transmitter the amplitude of by means of .a signal of comparatively small the oscillations is made to vary in accordance power. l with the signal to be transmitted. When the A further object of the present invention is input signal is of low amplitude, so that a low to provide a novel scheme and means for impercentage of modulation of the carriers is acpressing on high frequency oscillations phase 15 complished, the output of the transmitter is variations truly representative of the signalso less than when the signal is oi high amplitude that a good quality modulation is obtained and and a larger percentage of `modulation is imto accomplish the same by the usev of a minpressed on the carrier. With amplitude modimum amount of apparatus operating extremely ulation the transmitter must be operated below eiciently. o

` `full power output to be able to follow the sig- The above objects are obtained in accordance nal modulation to its peak value. When using with the present invention by modulating osamplitude modulation the power output of the cillations of constant frequency at the source transmitter variesin accordance with the perof production, that is, in the oscillator. For

cent modulation. In order that Athe transmitter example, if a crystal oscillator is used to pro- 25 may be capable of carrying full output on high duce constant frequency oscillations, modulapercent of modulation it must be run at a value tion may be accomplished in the crystal oscilbelow its full output rating on the average lator. In a crystal oscillator the crystal voltmodulation. For example, when a broadcast age works into the grid to cathode capacity and program is sent out over the usual broadcast a resistive component caused by the grid to transmitter the average modulation is about cathode current of the tube. This is the input 30%. `The transmitter must be so adjusted impedance of the tube and, as thegrid current that while it only delivers a small amount of is caused to vary by changing the applied grid its power on the average modulation it can voltage, that is, by changing the potential ap- 5l carry the peaks or the louder parts of the sigplied to the grid 0f. the Oscillator at Signal 35 nal without distortion. Thus the transmitter frequency, the grid to cathode resistive commust be operated below its full output most of ponent varies and the variations produce a the time and less power is radiated than would phase shift proportional to the variations of be possible with this transmitter. In phase the grid to cathode resistance.

40 modulation, however, the phase relation of the Arly tube that dIaWS Sufficient grid t0 Cathode 40 carrier and the sidebands varies in such a mancurrent, so that the resistive component is sufner that the resultantoutput remains substancient, may be modulated in this manner, tially constant. In phasermodulation the outwhether the tube be an oscillator or an amput of the transmitter remains substantially Dlel- Irl 2' mOdCatOn 0f the present IIVGII- constant and the modulation sidebands and tion I provide anew and efficient means for 45 carrier vary in such a manner that the output producing phase variations at signal frequency energy remains substantially constant. A in oscillations repeated in any thermionic relay.

phase modulated transmitter may beY operated The novel features of my invention have been at maximum output constantly. This results pointed out with particularity in the claims apin increased, efciency and permits more radiapended hereto. Y 50 tion power when using equipment of the same My novel method of producing phase variapower rating. The oscillations modulated in tions in oscillations which are characteristic of phase as indicated above `may be altered in signal currents, and devices for accomplishing characteristics at signal frequency in any of the same will be described in detail hereinafter.

` the succeeding stages prior to transmission, In this description reference will be made to 55 Figures la, 3, 3a, 4, 5 and 6 show modifications of the arrangement of Figure 1.

A specific embodiment of the invention will now be described. In describing said embodiment reference will be made to Figure 1 of the drawings in which, for purposes of illustration, an oscillator and phase modulating means, arranged in accordance with the present invention, have been shown.

In Figure 1 a thermionic tube C has its anode 4 connected as shown to a parallel circuit 6 tuned to a frequency slightly above the frequency it is desired to produce, the circuit 6 being inductive in character. The control grid 2 of C is connected to ground by way of a leak resistance 20 and a piezo-electric crystal PC, as shown. Due to the capacity between the electrodes 2 and 4 and the impedance of the tuned circuit 6 oscillations of constant frequency are produced in C and its associated circuits when the electrodes of C are energized. The frequency of the oscillations is determined by the value of the elements included in said circuits, and in particular by the physical dimensions of the piezo-electric crystal PC. The

anode to cathode and grid to cathode alternating current circuits are completed by connecting the cathode 8 to ground by way of inductance I 0, as shown, and by connecting the lower potential terminal of 6 to ground by way of a bipass condenser C1, as shown. The cathode electrode 8 is heated by means of a lamentary heating element and heating circuit which may be energized from any source not shown by way of a transformer 2|. To prevent high frequency oscillations from reacting on this transformer each side of the filament heating circuit is con nected to ground by way of capacity C2, as shown. In operation the circuit 6 is tuned to a frequency slightly above the natural frequency of the crystal PC. The external impedance of the plate circuit is high. The resistance 20 furnishes the eiective grid bias for the oscillator due to grid rectification. A positive potential may be applied to the anode electrode 4 by way of a tuned circuit 6, which may be connected with the positive terminal of any source of constant direct current potential the negative terminal of which is grounded. The arrangement just described Will, as is known, produce oscillations of constant frequency, which frequency will be determined in part by the piezo-electric crystal PC, and in part by the tuned circuit 6. The oscillations so produced may be impressed from circuit 6 on additional apparatus 38, and from 30 to a utilization circuit 32.

The manner in which the oscillations produced in C are modulated in phase at signal frequency will now be described. A source of ground to the terminals of a reactor lll. The potentials at signal frequency, appearing across ID, are applied to the cathode 8 of tube C. In order that radio frequency oscillations produced in C do not react on the oscillations in the reactor l0 or source A or both, the cathode is connected to ground by way of a radio frequency b-pass condenser C1. The condenser C1 forms a low impedance path by which the radio frequency current may return to the cathode Without passing through the modulator, but is of high impedance to the modulating poten- Y tials. The potentials applied at signal frequency to the cathode 8 vary the potential of the cathode with respect to the grid electrode 2 and thereby vary the impedance between the grid and cathode of said tube and, consequently, vary the grid to cathode current. Since the crystal voltage is working into the grid to cathode capacity, and the resistive component caused by the grid to cathode direct current, this variation of potential and of current results in a phase shift which is proportional to the variation of the resistive component in the grid to cathode circuit. This in turn phase modulates the current in the anode circuit of the oscillator C so that high frequency oscillations, modulated in phase in accordance with the signal, appear on the anode electrode 4 and are impressed therefrom to the tuned circuit 6. The oscillations so produced and modulated may be passed through an amplitude limiter or a frequency multiplier or an amplifier, or one or more such devices, all of which may be included in the unit 30. The phase modulated oscillations modified as desired may be utilized in any manner. For example, they may be impressed on a line for transmission or may be impressed, as shown, on an aerial system 32 for radiation. Furthermore, as indicated above the phase modulated oscillations may be further modulated in any known manner before transmission. This modulation may take place any place in the circuits, but preferably takes place after the phase modulation as described above is accomplished.

Although it is thought that the manner in which phase modulation has been accomplished will be understood from the above, a further brief explanation of what takes place in the circuits associated with C to produce phase modulation at signal frequency will be given.

Referring to Figure 2, G represents the quartz crystal input to tube C. The crystal may be considered as a constant voltage generator. Wc can make such an assumption for the crystal has so much stored energy (circulating current) that there is a persistency of oscillation greater than the audio frequency. If the crystal response curve (resonance curve) is about fifty cycles wide then any audio frequency above that will not effect the output voltage and will not react back on the crystals frequency of oscillation. This crystal output voltage e1 is impressed into an impedance network, made up of series elements and shunt elements. For purposes of illustration the series elements may be lumped, and Z represents the lumped series element while R and C represent the shunt elements. R consists mainly of the tube impedance while C is the inter-electrode and inter-circuit capacity. By varying the grid voltage (raising and lowering the cathode voltage) the resistive element of the shunt impedance is Varied. The amount of grid current through R and C changes and this changes the phase of the voltage across R and C and consequently the phase of the grid voltage in accordance with the input tone voltage ET.

In the Vector diagram of Fig. 2a, ei is the voltage developed by the crystal at G and applied to the network. e2 is the voltage at the input to the shunt impedances R and C and may represent the voltage applied to the grid of the oscillator for modulation purposes. Z is the impedance of the circuit, the tube electrodes etc. IT is the total grid current which iiows through Z. Ic is the current which flows through capacity C. IR is the current which flows through R. Iz is the potential drop through Z due to current IT. By mere inspection it may be seen that as we change the input impedance of the tube by varying R, the phase of the grid voltage e2 is changed. If R is changing, the phase of the network RC is changed, consequently e2 must change in phase. For example, if R is infinite, a large amount of the current ows through C and the circuit is capacitive and the phase of e2 is shifted, say ninety degrees relative to the current Z. If R is small, a large amount of the current flows through R and the network is resistive in character and e2 is in phase with the current through Z. VThe phase shift and other values given above are not given by way of limitation but are merely for purposes of illustration. The rule holds good for intermediate values of R, as well as the extreme values used. The current vector diagram adjacent the voltage vector diagram in Fig. 2a brings out veryk `of piezo-electric controlled oscillator circuit may be used to produce the oscillations which may in turn be modulated in phase in accordance with my novel scheme as illustrated in Figure l.

For example, an oscillation generator of the type disclosed in Figure 3 may be used to produce constant frequency oscillations of a constant amplitude.` This generator comprises a thermionic tube C having its control grid 2 connected, as shown, by Way of a grid condenser GC and a tuned circuit I2 to ground. The circuit I 2 may comprise capacity and inductance as shown. The anode electrode A mav be connected to a tuned circuit B and to a source of positive potential. The circuit 6 may include inductance and capacity, as shown. The anode electrode 4 may also be coupled to the grid electrode 2 by way of a piezo-electric crystal PC` as shown. Here the cathode 8 is heated in the same manner that the corresponding element is heated in the arrangement of Figure 9.. 'The proper biasing potentials are provided m thev control ,grid 2 due to the flow of rectified grid current in the resistance 20. Due to the impedance of the circuit 6 and to the coupling between the anode and grid electrodes in the tube C and external thereto by way of the piezo-electric crystal PC, sustained oscillations will be produced in the tube C. The frequency of these oscillations will be 'determined in part by the circuits 6 and I2 and in part by the natural frequency of the piezo-electric crystal PC.

The oscillations developed are impressed by way of the transformer on to the utilization circuit 32. Here, as in Figure l, the high frequency oscillations may be modulated at signal frequency by modulating potentials from the source A applied by way of the coupling capacity |6 and choking inductance I0 on to the cathode electrode 8. As pointed out in detail hereinbefore, these modulating potentials vary the impedance between the grid and cathode of said tube and, consequently, work on the grid to cathode direct current. Since the crystal voltage is working on the grid and on the resistive component caused by the grid to cathode direct current, the variations in potential at signal frequency, result in phase shifts of the high frequency oscillations produced in C which are proportional to the amplitudes of the modulating potentials and of a rate equal to the frequency of the modulating potentials.

In both arrangements described the modulating potentials applied from A may represent signals which may be demodulated in a receiver or the modulating potentials from A may be merely for the, purpose of improving the efficiency of transmission of the signals. In the latter case the phase wobbled high frequency oscillations may be keyed or otherwise modulated in subsequent stages with intelligible signals. The frequency modulated or wobbled signals may be amplitude limited and frequency multiplied and amplified or amplitude limited and frequency multiplied or amplitude limited, and amplified in the stage 3B and radiated from the aerial 32.

In some cases it may be desirable to modify the arrangements of Figures l and 3 as shown in Figures la and 3a. Here the parallel tuned circuit 6 has been replaced by a single inductance I. The inductance I may be wound so that, due to its distributed capacity, it will offer a very high impedance to the radio frequency currents. The use of such an inductance also eliminates the necessity of tuning the oscillator at this point. This has a two-fold advantage in Ythat the crystal oscillator may be set and, since there is no tuning in the circuit. the crystal will always oscillate under the same conditions of feedback and with the same voltage impressed on the grid circuit. If the tank tuning is changed, the feedback is changed and the voltage built up on the grid and impressed upon the crystal is also changed. Therefore, I supply a modulating voltage and calibrate the transmitter for a certain percentage modulation and the tank circuit tuning is changed, for example, by the tuned circuits in Figures l and 3. The percentage modulation will also be changed. Of course, vin practical operatio-n the arrangements of Figures l and 3 may be used successfully as it is a simple matter for a trained operator to retune the oscillator and adjust the circuits to produce the desired feedback and percentage modulation. In some cases, however, the modifications of Figures la and 3a, wherein the distributed capacity inductances I are utilized, may be p-referable.

In practice I use a small therm'onic tube C to produce the high frequency oscillations. When the tube is oscillating the rectified grid direct current fiowing in the grid leak resistance holds the control electrode of the tube at a certain operating point, which is determined by the circuit constants, such as, plate voltage, feedback voltage, etc. If, for some reason, the tube should stop oscillating as, for example, on failure of the crystal, the control grid of the tube C would be biased to a point at which a large amount of plate current would ow. This results from the fact that when oscillations cease rectified grid direct current no longer flows in the grid leak resistance and the control grid approaches ground. The high plate current will damage the tube under these circumstances or at least reduce the operating efficiency thereof.

To protect the tube an arrangement as shown in Figure 4 may be used. In this arrangement I place a resistor R1 in the cathode return of the plate circuit in series with the modulator choking inductance l0. This resistance is of sufficient value to oppose a surge of current in the plate circuit if the oscillator should stop operating and thereby to prevent damage to the tube C under such circumstances. The resistance R1, however, is not of sufficient value tohave noticeable effect upon the oscillator C and its circuit when the crystal PC is oscillating. This is because when oscillations are produced rectified direct current in the grid circuit builds up in the resistance Z a high negative bias which is applied to the grid electrode of tube 2 so that said tube, while oscillating, draws but a small amount of plate current. However, when the tube stops oscillating, the plate current at once approaches a much higher value due to the fact that the bias obtained by the grid leak bias elements in the grid circuit becomes lower. This high plate current across the cathode resistor R1 produces a voltage drop which biases the tube down to a safe value. The tube, therefore, is protected by means of a novel arrangement, which has little or no effect upon the operating characteristics of the oscillation generator.

The other features of the circuit of Figure 4 and the operation thereof, by means of which oscillations are produced and modulated in phase at signal frequency, will be understood by inspection of said figure and in view of the detailed description of the prior circuits and the operation of said circuits.

In the arrangements of Figures l, la and 4 production of oscillations is obtained by the nature of the plate circuit, the impedance thereof, and by the capacity between the electrodes in the tube.

In the arrangements of Figures 3 and 3a external coupling between the control grid and anode by way of the crystal PC is provided in addition to the internal couplings and the plate circuit impedance.

Where inductive coupling between the input and output electrodes is desired, arrangements as shown in Figures 5 and 6 may be used. In these arrangements the anode Aelectrode Il is inductively coupled by inductance 40 with an inductance :t connected between the grid 2 and the cathode of the tube C. The inductive coupling balances out the capacity feedback in the tube from plate to control grid and offers inductive feedback to the grid circuit so that sustained oscillations are produced. The piezo-electric resonator PC is connected in series with the grid cathode circuit, as shown. To prevent capacitive coupling between the tuned grid circuit and the plate the inductance 40 may be electrostatically shielded from the inductance il as shown. In this arrangement the high frequency oscillations produced in i he anode circuit feed energy back into the grid circuit to enhance said oscillations. The frequency of the oscillations produced is determined in part by the tuned circuit including the inductance 4I and the parallel variable condenser, and by the crystal PC. The oscillations may be modulated in phase by potentials from the modulating potential source as in the prior arrangements. The phase modulated oscillations may be fed by Way of a coupling capacity to the load circuit 32, either directly or by way of a unit 3U, which may include an amplitude limiter, a frequency multiplier and an amplier, and in one or more of said elements.

In Figure 6 the source of modulating potentials is connected by way of a transformer T to the cathode 8 rather than by Way of a choking inductance and blocking capacity, as shown in the prior figures. This arrangement may, in some cases, be preferable where the oscillations produced in C are to be phase modulated by telephony signals. The transformer coupling as shown in Figure 6 may, in some instances, give a better incoming characteristic over the various frequencies of the telephony frequency band than may be obtained by the condenser choking method of the prior figures. In some cases means, in addition to the capacity C1, may be necessary to prevent the high frequency oscillations from reaching the modulation potential source. This means may take the form of a radio frequency choke coil RFC, which may be con nected in series between the cathode 8 and the secondary winding of the transformer T by the switches shown. This choking inductance will force the high frequency oscillations to take the shorter path including the capacity C1.

While I have shown the modulating potentials applied to the cathode and control grid electrodes it will be understood that I contemplate the application of the modulating potentials to the plate electrode, and in case tubes of more than three electrodes are used to the screen grid electrode, or any other appropriate electrode.

Having thus described my invention and the operation thereof, what I claim is:

l. In phase modulated signal producing means, a thermionic vacuum tube having a control grid electrode, an anode and a cathode, frequency determining means connected between the control grid and cathode of said tube, a tunable circuit connected with the anode of said tube to produce oscillations in said tube and said frequency determining means, a leakage path between the grid electrode of said tube and ground, said leakage path including a resistance which determines the potential of said grid electrode relative to ground, a modulating potential impedance connecting the cathode of said tube to ground, and a source of modulating potentials connected to said last named impedance to vary the potential of said cathode relative to ground in accordance with said modulating potentials.

2. In a system for producing high frequency oscillations and for modulating the same in phase, a thermionic tube having an input circuit including, a quartz crystal provided with two electrodes, a connection between one of said electrodes and the control grid of said tube, a connection between the other of said crystal electrodes and ground, a tunable resonant circuit connected between the anode and cathode of said tube for obtaining therein and in said circuits oscillations of a frequency determined by the natural frequency of vibration of said quartz crystal, a source of modulating potentials, a capacity connecting the source of modulating potentials to the cathode of said tube, a modulating potential impedance connecting the cathode of said tube to ground, and an impedance connecting the control grid of said tube to ground.

3. In a system for producing phase modulated high frequency oscillations, a thermionic tube having an anode, a cathode and a control electrode, a piezo-electric crystal connected between said control electrode and ground, a leak resistance connected in parallel with said crystal, a tunable resonant circuit connected between said anode and ground, an inductance connected between said cathode and ground, and a source of modulating potentials connected in parallel with said inductance.

4. In a phase modulated wave signalling system, a thermionic tube' having an anode, a cathode and a control electrode, a frequency determining circuit including a piezo-electric crystal connected between the control electrode of said tube and ground, inductive means in a circuit connected between the anode of said tube and ground, an impedance connecting said cathode to ground, a source of modulating potentials connected in parallel to said impedance, and a load circuit coupled to said anode circuit.

5. Signalling means comprising an oscillation generator including, a thermionic tube having an anode, .a cathode, and a control grid, a piezoelectric element in circuit with the control grid Vand another electrode of said tube, an inductance connected between the anode and cathode of said tube, sources of potential for energizing the electrodes of said tube, whereby sustained oscillations are produced in said tube of a frequency determined in part by said piezo-electric crystal and in part by said inductance, means for maintaining the'cathode of said tube above ground audio frequency potential, and means for modulating the phase of the oscillations produced including a source of modulating potentials couped to the cathode of said tube and to ground.

6. In a system for producing high frequency oscillations modulated in phase, a thermionic` Vacuum tube having a control grid, an anode and a cathode, a piezo-electric crystal in a circuit connected between the control grid and cathode of said tube, a connection between a point on said circuit and ground, an inductance having distributed capacity connected with the anode of said tube to produce oscillations in said tube and said piezo-electric crystal, a leakage path between the grid electrode and cathode of said tube, said leakage path including a resistance which determines' the potential of said grid electrode relative to said cathode, and means for Varying the potential of the cathode of saidtube relative to ground at signal frequency to thereby vary the phase of the oscillations produced.

7. Means for producing high frequency oscillations and for modulating the same in phase comprising a thermionic tube having an input circuit including, a quartz crystal provided with two electrodes, a connection between one of said electrodes and the control grid of said tube, another connection between the other of said crystal electrodes and ground, a tunable resonant circuit comprising parallel inductance and capacity connected between the anode and cathode of said tube for obtaining therein and in said circuits oscillations of a frequency determined by the natural frequency of vibration of said quartz crystal, a source of modulating potentials, a capacity and a resistance connecting the source of modulating potentials to the cathode of said tube, and an inductance connecting the cathode of said tube to ground.

8. Oscillation producing means comprising, a thermionic tube having an anode, a cathode and a control grid, a frequency determining circuit including a piezo-electric crystal connected with the control grid of said tube, resistive means connecting the control grid to ground, inductive means of considerable distributed capacity connected between the anode of said tube and ground, the inductance and distributed capacity being the sole tuning means for said anode circuit, an inductance connecting said cathode to ground, and a source of modulating potentials connected in parallel to said inductance.

9. Signalling means comprising an oscillation generator including, a thermionic tube having an anode, a cathode, and a control grid, a piezoelectric element in circuit with the control grid and cathode of said tube, an inductance in a circuit between the anode and cathode of said tube, means for regeneratively coupling said circuits, means for energizing the electrodes of said tube, whereby sustained oscillations are produced in said tube of a frequency determined in part by said piezo-electric crystal and in .part by said inductance, and means for modulating the phase of the oscillations produced including a source of modulating potentials, a connection between said first circuit and ground, and a transformer having a primary winding connected with said source of modulating potentials and a secondary winding connected at one terminal through an inductance to the cathode of 'said tube and at the other terminal to ground.

10. Means for producing oscillations and for phase displacing the same in accordance with modulating potentials comprising, a thermionic tube having a control grid, a cathode and an anode, a circuit for heating said cathode, a piezoelectric crystal in a circuit in series with the control grid and with a parallel tuned circuit, the other terminal of which is connected to ground, a grid leak resistance and an additional impedance connected between the control grid and ground, an anode circuit including an inductance and a source of potential connected to the anode of said tube, said inductance being inductively couped to said tuned circuit, whereby sustained oscillations are produced in said tube and in said circuits, a source of modulating potentials connected between the cathode of said tube and ground, and an impedance in parallel with said source of modulating potentials.

11. In a system for producing phase modulated waves, a therrnionic tube having an anode, a cathode and a control grid, a piezo electric crystal connected to said control grid, a reactance connected to the anode of said tube and to the cathode of said tube, said reactance being inductive at the natural frequency of said piezo-electric crystal whereby oscillations are produced in said tube when the electrodes thereof are energized, an impedance connecting the control grid of said tube to ground, an audio frequency reactance connecting the cathode of said tube to ground, and a source of modulating potentials connected the the cathode of said tube and to ground to vary the potential of the cathode of said tube relative to ground potential. l

12. In a system for producing phase modulated waves, a thermionic tube having anode, cathode and control grid electrodes, a piezo electric crystal connected in circuit between said control grid and another electrode of said tube, reactive means connected in circuit with the anode of said tube, there being coupling between the control grid and anode of said tube, whereby oscillations are produced in said tube and circuits When the electrodes thereof are energized, a resistance connecting said control grid to ground, and a source of modulating potential connected between the cathode of said tube and ground to Vary the potential of said cathode relative to ground at signal frequency,

13. In a system for producing phase modulated waves, a thermionic tube having an anode, a cathode and a control grid, a circuit connected between the control grid and cathode of said tube, a connection between a point on said circuit and ground, a frequency determining element connected to said circuit, a reactance connected to the anode of said tube and to the cathode of said tube, said reactance being tuned toi a frequency slightly higher than the natural frequency of said piezo electric crystal whereby oscillations are produced in said tube when the electrodes thereof are energized, an audio frequency reactance connected between the cathode of said tube and ground, and a source of modulating potentials connected to the cathode of said tube and to ground.

14. In a system for producing phase modulated waves, a thermionic tube having an anode, a cathode and a control grid, a piezo-electric crystal connected in a circuit between said control grid and said cathode, a resistance in parallel with said crystal, a connection between said circuit and ground, a reactance connected with the anode of said tube, said reactance being inductive at the natural frequency of said piezoelectric crystal, whereby oscillations are produced in said tube when the electrodes thereof are energized, an audio frequency reactance and a resistance in series connecting the cathode of said tube to ground, and a source of modulating potentials and a capacity in seriesl connected in shunt to said audio frequency reactance.

15. In a system for producing oscillations modulated in phase in accordance with modulata ing potentials, a thermionic tube having an anode, a cathode, and a control grid, a piezoelectric element in circuit with the control grid and another electrode of said tube, an impedance connecting the control grid of said tube to ground, a parallel connected inductance and capacity constituting a tunable circuit connected with the anode of said tube, said circuit being adapted to be alsoS connected to the cathode of said tube, means for energizing the electrodes of said tube, whereby sustained oscillations are produced in said tube of a frequency determined in part by said piezo-electric crystal and in part by said tunable circuit, an impedance connected between the cathode of said tube and ground, and a source of modulating potentials connected in shunt toy a portion of said impedance to vary the potential of said cathode relative to ground in accordance with said modulating potentials,

16. Signalling means comprising an oscillation generator including, a thermionic tube having an anode, a cathode and a control grid, a piezo-electric crystal connected in circuit with certain of said electrodes to produce definite frequency control, an inductive impedance connecting the anode of said tube to the cathode of said tube and to a source of potential to produce high frequency oscillations in said tube, a leakage path between the control grid of said tube and ground, said leakage path including a resistance which determines the potential of said control grid electrode relative to ground, and means for modulating the phase of the oscillations produced in said tube by varying the input resistance of said tube including a device for applying signal potentials between the cathode of said tube and ground to vary the potential of said cathode relative to ground at signal frequency.

HALLAN EUGENE GOLDSTINE. 

